This invention relates to callback facilities in telecommunications systems. 2. Related Art
It is well known to provide a facility in a telephone system to allow a caller, on receiving an indication that a call attempt has failed, to be given the opportunity to request notification of the availability of the called party, so that he can make a further, hopefully more successful, attempt to place the call. The call failure mode may be “busy” (“off hook”), “no answer” (remains “on hook” despite ringing) or, in the case of a mobile telephone, “no contact” (switched off or not in range). The notification of the availability of the called party is typically triggered by the called party goes through the transition from an “off hook” state to an “on hook” state at some time subsequent to the request for such notification. In the case of a “no contact” situation, notification may be triggered by the called handset establishing contact with a cellular base station when it is switched on or comes into range.
Such systems are very useful, but have limitations because the calling party may not be in a position to place the desired call at the time the notification is received. This may be simply because the calling party may himself now be “off hook”, unavailable or switched off. In such a situation the callback may fail, but some systems are arranged to only attempt a “callback” when both parties are idle.
Systems of this general type are well-known—a typical example is described in International Patent specification WO00/22802 (Templeton).